UnitConverterViewModel - Windows Calculator

Overview

The UnitConverterViewModel class manages unit conversions for various categories (Currency, Volume, Length, Weight, Temperature, Energy, etc.). It handles real-time currency data, supplementary results, and bidirectional conversion between units. Namespace: CalculatorApp.ViewModel Implements: Windows::UI::Xaml::Data::INotifyPropertyChanged Declared in: UnitConverterViewModel.h:156

Observable Properties

Category and Units

CurrentCategory

property Category^ CurrentCategory
{
    Category^ get();
    void set(Category^ value);
}

The currently selected conversion category. When changed, updates the Units collection and resets the converter.

Units

OBSERVABLE_PROPERTY_R(
    Windows::Foundation::Collections::IObservableVector<Unit^>^,
    Units
);

Collection of available units for the current category.

Mode

OBSERVABLE_PROPERTY_RW(CalculatorApp::ViewModel::Common::ViewMode, Mode);

The current ViewMode (Currency, Volume, Length, etc.).

Input/Output Values

Unit1

OBSERVABLE_PROPERTY_RW(Unit^, Unit1);

The first (source) unit for conversion.

Value1

OBSERVABLE_PROPERTY_RW(Platform::String^, Value1);

The value in the first unit (user input).

Value1Active

OBSERVABLE_PROPERTY_RW(bool, Value1Active);

Indicates whether the first value field is currently active for input.

Unit2

OBSERVABLE_PROPERTY_RW(Unit^, Unit2);

The second (target) unit for conversion.

Value2

OBSERVABLE_PROPERTY_RW(Platform::String^, Value2);

The value in the second unit (conversion result or user input when active).

Value2Active

OBSERVABLE_PROPERTY_RW(bool, Value2Active);

Indicates whether the second value field is currently active for input.

Currency-Specific Properties

CurrencySymbol1

OBSERVABLE_PROPERTY_RW(Platform::String^, CurrencySymbol1);

Currency symbol for the first unit (e.g., ”$”, ”€”).

CurrencySymbol2

OBSERVABLE_PROPERTY_RW(Platform::String^, CurrencySymbol2);

Currency symbol for the second unit.

CurrencySymbolVisibility

property Windows::UI::Xaml::Visibility CurrencySymbolVisibility
{
    Windows::UI::Xaml::Visibility get();
}

Controls visibility of currency symbols. Returns Collapsed if either symbol is empty.

IsCurrencyCurrentCategory

OBSERVABLE_NAMED_PROPERTY_R(bool, IsCurrencyCurrentCategory);

Indicates whether the current category is Currency.

IsCurrencyLoadingVisible

OBSERVABLE_NAMED_PROPERTY_RW(bool, IsCurrencyLoadingVisible);

Controls visibility of the currency data loading indicator.

CurrencyDataLoadFailed

OBSERVABLE_NAMED_PROPERTY_RW(bool, CurrencyDataLoadFailed);

Indicates whether currency data failed to load from the network.

CurrencyDataIsWeekOld

OBSERVABLE_NAMED_PROPERTY_RW(bool, CurrencyDataIsWeekOld);

Indicates whether the currency data is more than one week old.

CurrencyRatioEquality

OBSERVABLE_PROPERTY_RW(Platform::String^, CurrencyRatioEquality);

Displays the exchange rate ratio (e.g., “1 USD = 0.85 EUR”).

CurrencyRatioEqualityAutomationName

OBSERVABLE_PROPERTY_RW(Platform::String^, CurrencyRatioEqualityAutomationName);

Accessibility-friendly version of the currency ratio.

CurrencyTimestamp

OBSERVABLE_PROPERTY_RW(Platform::String^, CurrencyTimestamp);

Displays when currency rates were last updated.

NetworkBehavior

OBSERVABLE_NAMED_PROPERTY_RW(
    CalculatorApp::ViewModel::Common::NetworkAccessBehavior,
    NetworkBehavior
);

Controls network access behavior for currency updates:

Normal - Fetch from network
Offline - Use cached data only
OptIn - Ask user permission

Supplementary Results

SupplementaryResults

OBSERVABLE_NAMED_PROPERTY_R(
    Windows::Foundation::Collections::IObservableVector<SupplementaryResult^>^,
    SupplementaryResults
);

Collection of suggested conversions to other related units. Example: When converting meters to feet, might show kilometers, centimeters, inches.

SupplementaryVisibility

property Windows::UI::Xaml::Visibility SupplementaryVisibility
{
    Windows::UI::Xaml::Visibility get();
}

Returns Visible if supplementary results are available, otherwise Collapsed.

UI State Properties

IsDecimalEnabled

OBSERVABLE_PROPERTY_RW(bool, IsDecimalEnabled);

Enables/disables the decimal point button.

IsDropDownOpen

OBSERVABLE_PROPERTY_RW(bool, IsDropDownOpen);

Indicates whether the unit selection dropdown is open.

IsDropDownEnabled

OBSERVABLE_PROPERTY_RW(bool, IsDropDownEnabled);

Enables/disables the unit selection dropdown.

Accessibility

Value1AutomationName

OBSERVABLE_PROPERTY_RW(Platform::String^, Value1AutomationName);

Accessibility text for the first value field.

Value2AutomationName

OBSERVABLE_PROPERTY_RW(Platform::String^, Value2AutomationName);

Accessibility text for the second value field.

Unit1AutomationName

OBSERVABLE_PROPERTY_RW(Platform::String^, Unit1AutomationName);

Accessibility text for the first unit.

Unit2AutomationName

OBSERVABLE_PROPERTY_RW(Platform::String^, Unit2AutomationName);

Accessibility text for the second unit.

Announcement

OBSERVABLE_PROPERTY_RW(
    CalculatorApp::ViewModel::Common::Automation::NarratorAnnouncement^,
    Announcement
);

Narrator announcements for conversion results.

Commands

CategoryChanged

COMMAND_FOR_METHOD(CategoryChanged, UnitConverterViewModel::OnCategoryChanged);

Triggered when the user selects a different conversion category.

UnitChanged

COMMAND_FOR_METHOD(UnitChanged, UnitConverterViewModel::OnUnitChanged);

Triggered when the user selects a different unit.

SwitchActive

COMMAND_FOR_METHOD(SwitchActive, UnitConverterViewModel::OnSwitchActive);

Swaps the input and output fields.

ButtonPressed

COMMAND_FOR_METHOD(ButtonPressed, UnitConverterViewModel::OnButtonPressed);

Handles number pad button presses.

CopyCommand

COMMAND_FOR_METHOD(CopyCommand, UnitConverterViewModel::OnCopyCommand);

Copies the conversion result to the clipboard.

PasteCommand

COMMAND_FOR_METHOD(PasteCommand, UnitConverterViewModel::OnPasteCommand);

Pastes a value from the clipboard.

Key Methods

Constructors

UnitConverterViewModel();

Default constructor.

internal: UnitConverterViewModel(
    const std::shared_ptr<UnitConversionManager::IUnitConverter>& model
);

Constructor with custom conversion model (internal use).

Public Methods

OnPaste

void OnPaste(Platform::String^ stringToPaste);

Processes pasted text as numeric input.

RefreshCurrencyRatios

void RefreshCurrencyRatios();

Manually triggers a refresh of currency exchange rates from the network.

OnValueActivated

void OnValueActivated(IActivatable^ control);

Activates the specified value field for input.

AnnounceConversionResult

void AnnounceConversionResult();

Announces the conversion result to screen readers.

SaveUserPreferences

void SaveUserPreferences();

Saves the current category and selected units to local settings for restoration on next launch.

RestoreUserPreferences

void RestoreUserPreferences();

Restores the previously saved category and units from local settings.

Internal Methods

UpdateDisplay

void UpdateDisplay(const std::wstring& from, const std::wstring& to);

Updates the display with converted values (called by the conversion engine).

UpdateSupplementaryResults

void UpdateSupplementaryResults(
    const std::vector<std::tuple<std::wstring, UnitConversionManager::Unit>>& suggestedValues
);

Updates the supplementary results collection.

OnMaxDigitsReached

void OnMaxDigitsReached();

Handles the maximum digit limit event.

OnCurrencyDataLoadFinished

void OnCurrencyDataLoadFinished(bool didLoad);

Callback when currency data loading completes.

OnCurrencyTimestampUpdated

void OnCurrencyTimestampUpdated(_In_ const std::wstring& timestamp, bool isWeekOld);

Callback when currency timestamp is updated.

Helper Classes

Unit

public ref class Unit sealed : public Windows::UI::Xaml::Data::INotifyPropertyChanged
{
public:
    property Platform::String^ Name { Platform::String^ get(); }
    property Platform::String^ AccessibleName { Platform::String^ get(); }
    property Platform::String^ Abbreviation { Platform::String^ get(); }
    bool IsModelUnitWhimsical();
    int ModelUnitID();
};

Represents a conversion unit.

SupplementaryResult

public ref class SupplementaryResult sealed : public Windows::UI::Xaml::Data::INotifyPropertyChanged
{
public:
    OBSERVABLE_PROPERTY_R(Platform::String^, Value);
    OBSERVABLE_PROPERTY_R(CalculatorApp::ViewModel::Unit^, Unit);
    bool IsWhimsical();
    Platform::String^ GetLocalizedAutomationName();
};

Represents a supplementary conversion result.

INotifyPropertyChanged Implementation

The ViewModel uses the OBSERVABLE_OBJECT_CALLBACK macro:

OBSERVABLE_OBJECT_CALLBACK(OnPropertyChanged);

This generates automatic property change notifications and calls OnPropertyChanged for custom handling.

Usage Examples

Basic Unit Conversion

// Create ViewModel
auto converter = ref new UnitConverterViewModel();

// Set category to Length
converter->CurrentCategory = lengthCategory;

// Set units
converter->Unit1 = metersUnit;
converter->Unit2 = feetUnit;

// Set value to convert
converter->Value1 = "10";

// Get converted value
Platform::String^ result = converter->Value2;
// e.g., "32.8084"

// Get supplementary results
auto supplementary = converter->SupplementaryResults;
// e.g., [{"10000", "Millimeters"}, {"0.01", "Kilometers"}]

Currency Conversion

// Create ViewModel
auto converter = ref new UnitConverterViewModel();

// Set to Currency mode
converter->Mode = ViewMode::Currency;
converter->CurrentCategory = currencyCategory;

// Select currencies
converter->Unit1 = usdUnit;
converter->Unit2 = eurUnit;

// Convert 100 USD
converter->Value1 = "100";

// Get result
Platform::String^ euros = converter->Value2;
Platform::String^ symbols = converter->CurrencySymbol1 + " to " + converter->CurrencySymbol2;
Platform::String^ ratio = converter->CurrencyRatioEquality;
// e.g., "1 USD = 0.85 EUR"

// Refresh currency rates
converter->RefreshCurrencyRatios();

// Check if data is old
if (converter->CurrencyDataIsWeekOld)
{
    // Show warning
}

Switching Active Fields

// User types in Value1 by default
converter->Value1Active = true;
converter->Value1 = "50";

// Switch to allow input in Value2
converter->SwitchActive->Execute(nullptr);

// Now Value2 is active, Value1 updates automatically
converter->Value2 = "100";
Platform::String^ reversedValue = converter->Value1;

XAML Binding Example

<ComboBox ItemsSource="{x:Bind ViewModel.Categories}"
          SelectedItem="{x:Bind ViewModel.CurrentCategory, Mode=TwoWay}" />

<ComboBox ItemsSource="{x:Bind ViewModel.Units}"
          SelectedItem="{x:Bind ViewModel.Unit1, Mode=TwoWay}" />

<TextBox Text="{x:Bind ViewModel.Value1, Mode=TwoWay}"
         AutomationProperties.Name="{x:Bind ViewModel.Value1AutomationName}" />

<TextBlock Text="{x:Bind ViewModel.CurrencySymbol1}" />

<ListView ItemsSource="{x:Bind ViewModel.SupplementaryResults}"
          Visibility="{x:Bind ViewModel.SupplementaryVisibility}" />

<Button Command="{x:Bind ViewModel.SwitchActive}" Content="⇄" />
<Button Command="{x:Bind ViewModel.CopyCommand}" Content="Copy" />

Persistence

The ViewModel automatically saves and restores user preferences:

// Save on category/unit change
converter->SaveUserPreferences();

// Restore on app launch
converter->RestoreUserPreferences();

Stored preferences include:

Last selected category
Last selected units for each category

Currency Data Management

Currency rates are fetched from the network and cached:

Initial load shows loading indicator (IsCurrencyLoadingVisible)
Data is cached locally with timestamp
If data is over 1 week old, CurrencyDataIsWeekOld is set
If network fails, CurrencyDataLoadFailed is set
User can manually refresh with RefreshCurrencyRatios()

Accessibility Features

Separate automation names for all values and units
Live announcements for conversion results
Support for whimsical units (special units with unique formatting)
Localized number formatting respects regional settings

Core Components

ViewModels

Unit Conversion

Graphing

​Overview

​Observable Properties

​Category and Units

​Categories

​CurrentCategory

​Units

​Mode

​Input/Output Values

​Unit1

​Value1

​Value1Active

​Unit2

​Value2

​Value2Active

​Currency-Specific Properties

​CurrencySymbol1

​CurrencySymbol2

​CurrencySymbolVisibility

​IsCurrencyCurrentCategory

​IsCurrencyLoadingVisible

​CurrencyDataLoadFailed

​CurrencyDataIsWeekOld

​CurrencyRatioEquality

​CurrencyRatioEqualityAutomationName

​CurrencyTimestamp

​NetworkBehavior

​Supplementary Results

​SupplementaryResults

​SupplementaryVisibility

​UI State Properties

​IsDecimalEnabled

​IsDropDownOpen

​IsDropDownEnabled

​Accessibility

​Value1AutomationName

​Value2AutomationName

​Unit1AutomationName

​Unit2AutomationName

​Announcement

​Commands

​CategoryChanged

​UnitChanged

​SwitchActive

​ButtonPressed

​CopyCommand

​PasteCommand

​Key Methods

​Constructors

​Public Methods

​OnPaste

​RefreshCurrencyRatios

​OnValueActivated

​AnnounceConversionResult

​SaveUserPreferences

​RestoreUserPreferences

​Internal Methods

​UpdateDisplay

​UpdateSupplementaryResults

​OnMaxDigitsReached

​OnCurrencyDataLoadFinished

​OnCurrencyTimestampUpdated

​Helper Classes

​Category

​Unit

​SupplementaryResult

​INotifyPropertyChanged Implementation

​Usage Examples

​Basic Unit Conversion

​Currency Conversion

​Switching Active Fields

​XAML Binding Example

​Persistence

​Currency Data Management

​Accessibility Features

​See Also

Build docs developers (and LLMs) love